This invention relates generally to a medical imaging system, and more particularly to a method and apparatus for registering 3D models of anatomical regions with projection images of the same.
During a number of interventional procedures related to the improvement of electrical therapy in the heart, the physician has to manipulate catheters and/or leads inside the heart chambers. An example of the two most complex and common procedures include atrial fibrillation (AF) ablation, and bi-ventricular pacing.
Atrial fibrillation, an arrhythmia in which the atria (upper chambers of the heart) stop contracting as they fibrillate, is the most common of the heart rhythm problems. In United States alone it is estimated that there are over 2 million people who have atrial fibrillation. Present data suggest that it is the most common arrhythmia-related cause of hospital admissions. Patients with AF tend to have a high incidence of such complications as stroke and congestive heart failure. Premature atrial contractions may act as triggers and initiate paroxysms of AF. These premature atrial contractions have been shown to predominantly originate in the pulmonary veins from the left atrium. Since infrequent and non reproducible premature atrial contractions may limit the utility of ablating trigger sites, a variety of surgical and catheter techniques have been used to isolate the pulmonary veins from the left atrium.
One of the surgical techniques used to treat (ablate) AF involves applications of radiofrequency waves to create small scars on the heart's surface near the connection between the pulmonary veins and the left atrium. The small scars created by the radiofrequency waves tend to stop the erratic impulses of atrial fibrillation by directing the impulses to follow a normal electrical pathway through the heart. Typically, this type of surgical procedure is performed through a chest incision. Surgeons use specially designed instruments to deliver radiofrequency waves to the abnormal tissue, typically during the open heart surgery performed for other reasons, such as valve surgery or bypass surgery for example. Although this type of surgical technique is effective when the patient is undergoing open heart surgery for another reason, catheter-related treatment methods are more practical when the patient does not require the invasive open heart surgery for other reasons.
One of the catheter techniques involves fluoroscopic guided positioning of catheter in the left atrium after going through a blood vessel, and the application of radiofrequency energy at areas showing double potentials suggestive of sites capable of conducting between the left atrium and the pulmonary veins. It has also been shown that ablation at other sites such as between the mitral valve and left pulmonary veins, and between the pulmonary veins, as is done during the surgical intervention, may increase the success rate of AF ablation. The three-dimensional reconstruction of the left atrium using some currently available technologies, the inability of the physician to visualize the pulmonary vein ostia (opening of these veins into the left atrium) from inside, the varying size of the pulmonary veins and thus the pulmonary vein ostia, the difficulty in keeping the mapping and ablation catheters stable at the pulmonary vein ostial site due to the complex 3D geometry of these areas, all make current approaches to mapping and ablation using current fluoroscopically guided techniques somewhat cumbersome and lengthy. Because of these limitations, surgery has been preferred over radiofrequency catheter ablation, especially in patients with persistent atrial fibrillation, and it is estimated that less than 20 percent of patients with persistent AF undergoing radiofrequency ablation for AF, benefit from this approach.
A factor that may be associated with the above mentioned limitation is that the operator typically guides an interventional tool using mainly the fluoroscopy images. A typical task in such a procedure is the placement of a catheter at a specific location, such as one of the pulmonary veins for example. These anatomical structures are not well depicted by the x-ray system since they do not present contrast versus the surrounding anatomical structures.
The medical task would be much easier if these target anatomical structures were visible in the fluoroscopy image in the precise anatomic fashion separate from the surrounding anatomic structures.
Another important procedure, as mentioned above, involves bi-ventricular pacing in the treatment of heart failure. Despite considerable progress in the management of congestive heart failure (CHF), it remains a major health problem worldwide. It is estimated that there are 6-7 million people with CHF in the United States and Europe, and approximately 1 million patients are diagnosed with CHF every year.
Despite significant advances in the treatment of CHF using various pharmacological therapies, quality-of-life in patients with CHF is poor as they are frequently hospitalized, and heart failure is a common cause of death. In addition, there is significant cost attached to this problem.
Normal electrical activation in the heart involves activation of the upper chambers, called the atria, followed by simultaneous activation of both the right and the left lower chambers, called the ventricles, by the left and right bundle branches. As patients with advanced CHF may have conduction system disease, which may play a role in worsening cardiac function, pacing therapies have been introduced in an attempt to improve cardiac function. One frequently noted conduction abnormality is left bundle branch block (LBBB). In one study, (Xiao HB, et al. Differing effects of right ventricular pacing and LBBB on left ventricular function. Br Heart J 1993; 69:166-73) 29% of patients with CHF had LBBB. Left bundle branch block delays left ventricular ejection due to delayed left ventricular activation as the electrical impulse has to travel from the right to the left side leading to sequential rather than simultaneous activation, as mentioned before. In addition, different regions of the left ventricle (LV) may not contract in a coordinated fashion.
Cardiac resynchronization, also knows as Bi-Ventricular (Bi-V) pacing, has shown beneficial results in patients with CHF and LBBB. During Bi-V pacing, both the right and the left ventricle (RV, LV) of the heart are paced simultaneously to improve heart pumping efficiency. It has also been shown recently that even some patients with no conduction system abnormalities, such as LBBB, may also benefit from the Bi-V pacing. During Bi-V pacing, in addition to the standard right atrial and right ventricular lead used in currently available defibrillators or pacemakers, an additional lead is positioned into the coronary sinus. The additional lead is then advanced into one of the branches of the coronary sinus overlying the epicardial (outer) left ventricular surface. Once all of the leads are in place, the right and left ventricular leads are paced simultaneously, thus achieving synchronization with atrial contraction.
There are, however, several problems with this approach. First, this type of procedure is time-consuming. Second, placement of the LV lead is limited to sites available that provide reasonable pacing and sensing parameters. And third, cannulating the coronary sinus may be challenging as a result of an enlarged right atrium, rotation of the heart, or presence of Tebesian valve (a valve close to the opening of the coronary sinus). Coronary sinus stenosis (occlusion) has also been reported in patients with prior coronary artery bypass surgery, further complicating the problem.
In most instances, problems with the placement of the coronary sinus lead are identified at the time of the interventional procedure. In the event of the coronary sinus lead placement procedure being abandoned, the patient is brought back to the operating room and the LV lead is positioned epicardially. During this procedure, an incision is made on the lateral chest wall and the lead is placed on the outer side of the left ventricle.
Unfortunately, there are many problems with epicardial lead placement as well, some of which include but are not limited to:
Limited view of the posterolateral area of the left ventricle using the incision of the chest wall, also called minithoracotomy;
The limited number of placement sites providing reasonable pacing and sensing parameters;
Inability to identify the most appropriate location and placement of the lead at the most appropriate site;
Potential risk of damaging the coronary arteries and venous system; and
Difficulty in identifying the ideal pacing site as a result of one or more of the above limitations.
It has also been shown that LV pacing alone may be as effective as Bi-V pacing. However, due to the unstable nature of the coronary sinus lead, a pacing and sensing lead is usually placed in the right ventricle in currently used techniques.
Cardiac CT may be used to create a roadmap of coronary sinus and left ventricular anatomy such that appropriate sites may be identified for the placement of a LV pacing lead for Bi-V/LV pacing, either at the most appropriate branch of the coronary sinus, or on the left ventricular wall epicardially (from outside). CT or MR imaging may also identify areas devoid of blood vessels and nerves, as well as scar tissue. These modalities may also be used to determine the asymmetric contraction of the ventricles and identify different regions of the ventricles not contracting in a coordinated fashion. The presence of scarring from previous heart attacks may make this uncoordinated contraction even worse.
While existing medical procedures may be suitable and appropriate for certain medical conditions, procedural limitations still exist. Accordingly, there remains a need in the art for an improved method and apparatus for registering 3D models of anatomical regions with projection images of the same to overcome these drawbacks.